Remove set-cookie header on custom.css (#16314)

* Remove set-cookie header on custom.css

* Additional fix for set-cookie

app/controllers/custom_css_controller.rb

@@ -3,11 +3,16 @@
 class CustomCssController < ApplicationController
   skip_before_action :store_current_location
   skip_before_action :require_functional!
+  skip_before_action :update_user_sign_in
+  skip_before_action :set_session_activity
+
+  skip_around_action :set_locale
 
   before_action :set_cache_headers
 
   def show
     expires_in 3.minutes, public: true
+    request.session_options[:skip] = true
     render plain: Setting.custom_css || '', content_type: 'text/css'
   end
 end